1. Field of the Invention
The present invention relates to a press-forming die set adapted to perform a pressing operation on a workpiece in the form of a thin sheet, so as to form a negative-angle portion on the workpiece. The press-forming die set includes a first die (e.g., an upper die and a second die (e.g., a lower die), which are moved toward and away from each other in a press-forming direction, usually, in the vertical direction. The workpiece is placed on the second die such that the inner surface of the workpiece is held in contact with the second die. This inner surface of the workpiece is referred to as "inner mounting surface". In the present application, the term "negative-angle portion" is interpreted to mean a portion of the press-formed workpiece which portion is bent with respect to the above-indicated press-forming direction such that the bent portion in question of the workpiece is inclined with respect to the press-forming direction, so as to generally extend under the inner mounting surface of the press-formed workpiece.
2. Discussion of Related Art
To form such a negative-angle portion on a thin sheet such as a thin metal or plastic sheet, there has been conventionally used a press-forming die set which includes a linear slide cam or a rotary cam. Where the linear slide cam is used to form the negative-angle portion, the linear slide cam is incorporated in a side guide cam provided on the lower die, and the upper die is vertically moved down toward the lower die on which the workpiece is placed, so that a downward movement of a cam fixed on the upper die causes a movement of the linear slide cam in the horizontal direction so as to bend a portion of the workpiece, for forming the negative-angle portion. After the formation of the negative-angle portion, the linear slide cam is moved away from the workpiece in a direction parallel to the surface of the formed negative-angle portion, which generally extends under the inner mounting surface of the workpiece. Thus, the negative-angle portion can be easily formed using the linear slide cam.
Where the negative-angle portion is formed on the workpiece by a press-forming die set including the linear slide cam, as described above, the linear slide cam is required to be moved in the direction parallel to the surface of the formed negative-angle portion, in order to remove the formed workpiece (product) from the die set. However, the negative-angle portion cannot be formed by the die set including the linear slide cam, where the width of the product obtained by press-forming the workpiece is relatively small, since the linear slide cam cannot be moved in the direction parallel to the formed negative-angle portion.
In a known press-forming die set including a rotary cam to form the negative-angle portion, on the other hand, the rotary cam having a negative-angle forming portion is incorporated in the lower die or in the side guide cam provided on the lower die, such that the rotary cam is rotatable about an axis parallel to the bottom surface of the lower die which is perpendicular to the press-forming direction. An example of such a press-forming die set is disclosed in JP-A-60-166122. Another example of such a known press-forming die including a rotary cam is shown in FIG. 6, which does not show an upper die and a lower die. In this die set, a workpiece 100 is placed on a guide cam 102 provided on the lower die, so that an inner mounting surface 104 of the workpiece 100 is held in contact with the upper surface of the guide cam 102, such that the inner mounting surface 104 is parallel to the bottom surface of the lower die. The upper die is adapted to be lowered toward the lower die in a direction perpendicular to the inner mounting surface 104. To bend the workpiece 100 so as to form a negative-angle portion 112, the rotary cam 106 is rotated in the clockwise direction "a" indicated in FIG. 6, so that a negative-angle forming portion 108 of the rotary cam 106 is moved toward a negative-angle forming portion 110 provided on the upper die, whereby the negative-angle forming portions 108 and 110 cooperate to form the negative angle portion 112 on the workpiece 100. After the formation of the negative-angle portion 112, the rotary cam 106 is rotated in the counterclockwise direction so that the negative-angle forming portion 108 is moved away from the negative-angle portion 112. In this case, therefore, the negative-angle portion 112 can be formed even where the width of the formed product having the negative-angle portion 112 is comparatively small, provided that the diameter of the rotary cam 106 can be made relatively small.
Where the negative-angle portion 112 is formed by the die set including the conventional rotary cam 106, this rotary cam 106 is required to be rotated so as to prevent an interference of its negative-angle forming portion 112 with the negative-angle portion 112 of the press-formed workpiece 100 (i.e., product), when the product is removed from the die set. For removing the product from the die set so as to prevent the interference, the angle of the movement path of the negative-angle forming portion 108 of the rotary cam 106 with respect to the inner mounting surface 104 of the workpiece 100 should decrease as the negative angle of the negative-angle portion 112 with respect to the pressforming direction increases. That is, when the negative-angle portion 112 generally extends almost parallel to the inner mounting surface 104, the negative-angle forming portion 108 is required to be moved away from the negative-angle portion 112 (negative-angle forming portion 110) in a direction almost parallel to the inner mounting surface 104 of the press-formed workpiece 100.
The angle of the movement path of the negative-angle forming portion 108 of the rotary cam 106 with respect to the inner mounting surface 104 decreases with an increase in the diameter of the rotary cam 106. Therefore, the diameter of the rotary cam 106 should increase with an increase in the negative angle of the negative-angle portion 112 with respect to the pressforming direction. However, it is difficult to incorporate the rotary cam 106 in the guide cam 102 where the diameter of the rotary cam 106 is relatively large. Further, a dimension "b" of the product (press-formed workpiece 100) decreases with an increase in the diameter of the rotary cam 106, so that the strength of the guide cam 102 decreases with an increase in the diameter of the rotary cam 106. Accordingly, the strength of the guide cam 102 is not sufficient when the width dimension of the product is comparatively small. Where the negative angle of the negative-angle portion 112 is excessively large, for instance, where the negative-angle portion 112 is almost parallel to the inner mounting surface 104, that is, almost perpendicular to the press-forming direction, the negative-angle forming portion 108 of the rotary cam 106 interferes with the inner mounting surface 104, namely, the path of movement of the negative-angle forming portion 108 intersects the inner mounting surface 104. In this instance, therefore, the rotary cam 106 cannot be rotated, unless the configuration of the product to be manufactured is suitably changed.